Ammonia sensor control, with nox feedback, of an scr aftertreatment system

ABSTRACT

An exemplary method includes determining an NH 3  reference target in an exhaust conduit between a first SCR catalyst and a second SCR catalyst. The method includes determining a present amount of NH 3  in the exhaust conduit between the first SCR catalyst and the second SCR catalyst, and determining an NH 3  error term in response to the NH 3  reference target and the present amount of NH 3 . The method further includes determining an amount of NO x  downstream of the second SCR catalyst, and adjusting one of the NH 3  reference target and a reductant doser command in response to the amount of NO x  downstream of the second SCR catalyst. The method further includes providing a reductant doser command in response to the NH 3  error term.

RELATED APPLICATIONS

This application is related to, and claims the benefit of, U.S.Provisional Patent Application No. 61/330,605 entitled AMMONIA SENSORCONTROL OF AN SCR AFTERTREATMENT SYSTEM and filed May 3, 2010, and U.S.patent application Ser. No. 13/051,693 entitled AMMONIA SENSOR CONTROLOF AN SCR AFTERTREATMENT SYSTEM and filed Mar. 18, 2011, both of whichare incorporated herein by reference for all purposes.

BACKGROUND

Control of SCR catalysts is of increasing interest to meet moderninternal combustion engine emissions standards. Feedforward controls aredesirable to maximize fuel economy, improve system responsiveness, andreduce undesirable emissions. However, feedforward control systems arenot capable of responding to disturbances that are not measured ormodeled in advance. Accordingly, further technological developments inthis area are desirable.

SUMMARY

One embodiment is a unique method for controlling an SCR system with aNO_(x) sensor feedback component. Further embodiments, forms, objects,features, advantages, aspects, and benefits shall become apparent fromthe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary system for control of an SCR aftertreatmentsystem.

FIG. 2 is an exemplary controller for executing operations to apply arate-based adjustment and/or a downstream NO_(x) adjustment to areductant doser command.

FIG. 3 is a schematic diagram of a control operation for applying arate-based adjustment and a downstream NO_(x) adjustment to an ammoniareference target.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, any alterations and further modificationsin the illustrated embodiments, and any further applications of theprinciples of the invention as illustrated therein as would normallyoccur to one skilled in the art to which the invention relates arecontemplated herein.

FIG. 1 is an exemplary system 100 for control of an SCR aftertreatmentsystem. The system 100 includes an internal combustion engine 102 and anexhaust conduit fluidly coupled to the internal combustion engine. Thesystem 100 includes a first selective catalytic reduction (SCR) catalyst108 fluidly coupled to the exhaust conduit, and a second SCR catalyst110 fluidly coupled to the exhaust conduit at a position downstream ofthe first SCR catalyst 108. The system 100 includes an ammonia sensor122 operationally coupled to the exhaust conduit at a position betweenthe first SCR catalyst 108 and the second SCR catalyst 110, and areductant doser 114 operationally coupled to the exhaust conduit at aposition upstream of the first SCR catalyst 108.

The system 100 further includes a controller 124 having a number ofmodules structured to functionally execute operations for controllingthe SCR system. In certain embodiments, the controller forms a portionof a processing subsystem including one or more computing devices havingmemory, processing, and communication hardware. The controller may be asingle device or a distributed device, and the functions of thecontroller may be performed by hardware or software. The controller maybe in communication with any sensor, actuator, datalink, and/or networkin the system.

In certain embodiments, the controller includes an NH₃ target module, anNH₃ determination module, an NH₃ error module, a dosing control module,an outer loop control module, a time response adjustment module, and/ora NO_(x) trimming module. The description herein including modulesemphasizes the structural independence of the aspects of the controller,and illustrates one grouping of operations and responsibilities of thecontroller. Other groupings that execute similar overall operations areunderstood within the scope of the present application. Modules may beimplemented in hardware and/or software on computer readable medium, andmodules may be distributed across various hardware or softwarecomponents. More specific descriptions of certain embodiments ofcontroller operations are included in the section referencing FIG. 2.

In certain further embodiments, the system 100 includes a dieseloxidation catalyst (DOC) 104 and a diesel particulate filter (DPF) 106positioned upstream of the first SCR catalyst 108. Any of thesecomponents may be present or missing, catalyzed or not catalyzed, andmay be arranged in alternate order. Further, certain components or allcomponents may be provided in the same or separate housings. The firstSCR catalyst 108 and the second SCR catalyst 110 may be positionedwithin the same catalyst brick, with the position of the ammonia sensor122 defining the separation point between the first SCR catalyst 108 andthe second SCR catalyst 110.

The reductant doser 114 is fluidly coupled to a reductant source such asa reductant storage tank 116. The reductant is any type of reductantutilized in an SCR aftertreatment system that results in ammonia beingutilized as the final reductant—including at least ammonia (gaseous oraqueous) and urea. Certain operations described herein apply to NO_(x)reduction generally and are not specific to SCR systems. Where theNO_(x) reduction operations are not specific to SCR systems, hydrocarbonor other reductants may be utilized.

The system 100 may include an ammonia oxidation catalyst (AMOX) 112downstream of the second SCR catalyst 110. In certain embodiments, theAMOX 112 may not be present, or the AMOX 112 may be commingled with thesecond SCR catalyst 110 (or the last SCR catalyst, where multiple SCRcatalysts are present), for example with a washcoat applied toward therear portion of the second SCR catalyst 110 that is responsive to atleast partially oxidize ammonia.

The exemplary system 100 further includes various sensors. Theillustrated sensors include a NO_(x) sensor 118 positioned upstream ofthe first

SCR catalyst 108, a second NO_(x) sensor 118 positioned downstream ofthe second SCR catalyst 110, a temperature sensor 120 positioned betweenthe first and second SCR catalysts 108, 110, and a temperature sensor120 positioned downstream of the AMOX catalyst 112. The illustratedsensors are exemplary only, and may be re-positioned, removed,substituted, and other sensors may be present that are not illustratedin FIG. 1. Certain embodiments of the system 100 do not include a NO_(x)sensor present upstream of the first SCR catalyst, an NH₃ sensor presentdownstream of the second SCR catalyst, or both. Further, certain sensorsmay instead be virtual sensors that are calculated from other parametersavailable to the system, or values that would be indicated by sensorsmay instead be supplied to a computer readable memory location, via adatalink or network communication, or otherwise be made available to thesystem where the sensor providing the sensed parameter is not a part ofthe defined system.

FIG. 2 is an exemplary controller 124 for executing operations to applya rate-based adjustment and/or a downstream NO_(x) adjustment to areductant doser command. The controller operations of the controller inFIG. 2 are operations that adjust nominal control operations for aNO_(x) aftertreatment system utilizing a reductant. Nominal controloperations for a NO_(x) aftertreatment system, including an SCRaftertreatment system, are understood in the art and are not describedfurther herein. Any nominal NO_(x) aftertreatment control operations maybe utilized, including without limitation control operations describedin U.S. Provisional application 61/330,605 “Ammonia sensor control of anSCR aftertreatment system,” filed May 3, 2010, and U.S. patentapplication Ser. No. 13/051,693 “Ammonia sensor control of an SCRaftertreatment sensor,” filed Mar. 18, 2011.

The controller 124 includes an ammonia (NH₃) target module 202 thatdetermines an NH₃ reference target 216. The NH₃ reference target 216 isa target amount of NH₃ in an exhaust conduit between the first SCRcatalyst and the second SCR catalyst. The NH₃ reference target 216,before adjustments, is a nominal control value for the controller,and/or is determined in response to the nominal control value. Forexample, the NH₃ reference target 216 may be the NH₃ value between theSCR catalysts that occurs in response to a targeted amount of reductantinjected upstream of the first SCR catalyst as determined from an amountof NO_(x) being emitted by the engine. In certain embodiments, the NH₃reference target 216 is the feedforward NH₃ concentration that isdesired at the position between the SCR catalysts according to a nominalcontrol algorithm before adjustments.

The controller 124 further includes an NH₃ determination module 204 thatdetermines a present amount of NH₃ 242 in the exhaust conduit betweenthe first SCR catalyst and the second SCR catalyst, and an NH₃ errormodule 206 that determines an NH₃ error term 218 in response to the NH₃reference target 216 and the present amount of NH₃ 242. The controller124 further includes a dosing control module 208 that provides areductant doser command 226 in response to the NH₃ error term 218. Thereductant doser command 226 provided by the dosing control module 208,before any adjustments from the outer look control module 210 areapplied, may be an inner loop reductant doser command 226.

The controller 124 further includes an outer loop control module 210that determines an amount of NO_(x) downstream of the second SCRcatalyst 230, and adjusts one of the NH₃ reference target 216 and thereductant doser command 226 in response to the amount of NO_(x)downstream of the second SCR catalyst 230. In certain embodiments, theouter loop control module 210 further adjusts the one of the ammoniareference target 216 and the reductant doser command 226 only when theamount of NO_(x) downstream of the second SCR catalyst 230 exceeds athreshold value. An exemplary threshold value is a NO_(x) referencetarget 220, which is a predetermined amount of NO_(x) that is determinedto be a high enough NO_(x) level that the outer loop control module 210should implement an adjustment and reduce the NO_(x) output of thesystem. In certain embodiments, the outer loop control module 210 is alow-gain control element relative to the inner loop control, and has acontrol response time on the same order of magnitude as a catalystdegradation rate is expected to occur in the system. However, the outerloop control module 210 may be responsive to other system disturbances,and may include control response times that are as fast as, or fasterthan, the inner loop control.

The outer loop control module 210 adjusts the NH₃ reference target, thereductant doser command, or both. One of skill in the art will recognizethat the adjustment of the NH3 reference target, the NH3 error term, orany other control parameter upstream of the reductant doser command 226ultimately adjusts the reductant doser command 226. The selection of anappropriate parameter for adjustment is a design choice, and theadjustment of any parameter that affects the reductant doser command226, including direct adjustment of the reductant doser command 226, iscontemplated herein. For example, where the outer loop control module210 determines that the amount of NOx downstream of the second SCRcatalyst 230 is too high, and is high enough to permit adjustment (e.g.exceeds the NOx reference target 220), the outer loop control module 210may increase the reductant doser command 226, increase the NH3 referencetarget 216, and/or increase the NH3 error term 218, where the increaseamount and time of the increase are selected to provide the desiredadjusted response of the reductant doser.

An exemplary controller 124 further includes an NH₃ time responseadjustment module 214 that determines at least one of a rate of changeof the present amount of NH₃ 232, and/or a rate of change of the amountof NO_(x) downstream of the second SCR catalyst 234. The NH₃ timeresponse adjustment module 214 further adjusts the one of the NH₃reference target 216 and the reductant doser command 226 in response tothe rate of change(s) 232, 234. A further exemplary controller 124includes the NH₃ time response adjustment module 214 determining aproduct 236 of the rate of change of the present amount of NH₃ and therate of change of the amount of NO_(x) downstream of the second SCRcatalyst, and further adjusting the one of the NH₃ reference target 216and/or the reductant doser command 226 in response to the product 236.

A further exemplary controller 124 includes the NH₃ time responseadjustment module 214 further determining a rate-based adjustment amount238 as a function of the product 236, and further adjusting the one ofthe NH₃ reference target 216 and the reductant doser command 226 inresponse to the rate-based adjustment amount 238. Additionally oralternatively, the function of the product 236 that determines therate-based adjustment amount 238 is a non-linear function. An exemplarynon-linear function includes a low gain in a negative region of theproduct and a high gain in a positive region of the product. The termlow gain and high gain implies that, for a given magnitude of theproduct, the value on the negative side is lower than the value on thepositive side. However, the response of the rate-based adjustment amount238 to the product 236 on each side of the product magnitude line may belinear, non-linear, a polynomial or other response function, or aselected function described from a lookup table having a desiredresponse curve.

In a further embodiment, the NH₃ time response adjustment module 214further limits the rate-based adjustment amount 238 to a proportion ofan amount of NO_(x) upstream of the first SCR catalyst 230. For example,the rate based adjustment amount 238 may be limited to an amount ofreductant increase (or decrease) that is sufficient to treat 10%, 25%,50%, or 100% of the incoming NOx amount 230. For example, if the amountof NO_(x) upstream of the first SCR catalyst 230 is 2 grams per minute,the rate-based adjustement amount 238 may be limited to sufficientreducant to treat 0.2 g/min, 0.5 g/min, 1.0 g/min, or 2 g/min ofincoming NOx. In certain embodiments, the rate-based adjustment amount238 may be unlimited, or limited to values that are higher thansufficient reductant to treat 100% of incoming NOx.

In certain embodiments, the apparatus includes a NO_(x) trimming module212 that determines an adjusted downstream NO_(x) amount 222 in responseto the amount of NO_(x) downstream of the second SCR catalyst 230 and anamount of NH₃ downstream of the second SCR catalyst 228. Determining theadjusted downstream NO_(x) amount 222 includes accounting forcross-sensitivity of NO_(x) sensors to NH₃, where the NO_(x) sensorerroneously interprets a portion of the NH₃ present as NO_(x) . In asimple embodiment, the detected amount of NH₃ 228 is subtracted from thedetected amount of NO_(x) 230 to determine the adjusted downstreamNO_(x) amount 222. Where the actual cross-sensitivity level is known,the correction may be utilized using the actual cross-sensitivitylevel—for example if 5 units of NH₃ are known to erroneously indicate 4units of NO_(x), then the adjusted downstream NO_(x) amount 222 may bedetermined by subtracting 80% of the NH₃ amount 228 from the NO_(x)amount 230.

In certain embodiments, the NO_(x) trimming module further adjusts theone of the NH₃ reference target 216 and the reductant doser command 226in response to the adjusted downstream NO_(x) amount 222. For example,where the adjusted downstream NO_(x) amount 222 indicates that a loweramount of NO_(x) is present than indicated by the NO_(x) amount 230, theNO_(x) trimming module may decrease the NH₃ reference target and/or thereductant doser command 226 accordingly. The decrease may be the amountof difference between the adjusted downstream NO_(x) amount 222 and theNO_(x) amount 230, or a fraction thereof. In further embodiments, theNO_(x) trimming module 212 determines an excess downstream NO_(x) amount224 in response to the adjusted downstream NO_(x) amount 222 and aNO_(x) reference target 220, and further adjust the one of the NH₃reference target 216 and the reductant doser command 226 in response tothe excess downstream NO_(x) amount 224. The NO_(x) reference target 220may be a regulated NO_(x) amount, a planned NO_(x) amount, or a NO_(x)amount determined according to criteria understood in the art.

The controller further includes a dosing control module 208 thatprovides the reductant doser command 226 in response to the NH₃ errorterm 218. The reductant doser command 226 may be provided under anycontrol scheme understood in the art, and/or under specific controlschemes described herein. The reductant doser command 226 may include anactuator command value, a voltage or other electrical signal, and/or adatalink or network command. In certain embodiments, a reductant doserin a system including the controller 124 is responsive to the reductantdoser command 226 to provide reductant to an exhaust stream at aposition upstream of an SCR catalyst.

In certain embodiments, the outer loop control module 210 determines anamount of NO_(x) downstream of the second SCR catalyst 230, and adjuststhe ammonia reference target 216 and/or the reductant doser command 226in response to the amount of NO_(x) downstream of the second SCRcatalyst 230. The outer loop control module 210 may operate on anexecution cycle that is much slower than the execution cycle of the NH₃target module 202. Additionally or alternatively, the outer loop controlmodule 210 may have limited authority to make adjustments to the NH3reference target 216 and/or the reductant doser command 226, either inmagnitude and/or rate of adjustment. For example, the outer loop controlmodule 210 may be limited to a maximum adjustment amount, or a maximumadjustment increment per selected unit of time. Further, the outer loopcontrol module 210 may be limited to making adjustments only when asignificant amount of NO_(x) is observed downstream of the second SCRcatalyst, for example only when the amount of NO_(x) downstream of thesecond SCR catalyst 230 exceeds a threshold. The threshold isdetermined, in one embodiment, on the amount of NO_(x) that must bepresent to provide an acceptable signal-to-noise ratio of the NO_(x)reading from the NO_(x) sensor downstream of the second SCR catalyst.

Further still, the outer loop control module 210 may filter or heavilyfilter the NO_(x) output observed downstream of the second SCR catalyst230 to reduce observed spikes or erroneous readings. Further still, theadjustment provided by the outer loop control module 210 may degradeover time back toward a non-adjusted value, or may reset under certainconditions to a non-adjusted value (e.g. when a service operation isperformed to change the first and/or second SCR catalysts).

In one embodiment, the outer loop control module 210 is utilized tocorrect for long-term damage or degradation to the first and/or secondSCR catalysts, and provides a direct feedback control element to thecontroller 124 that operates outside of the at least partiallyfeedforward NH₃ target module 202. Yet further still, the outer loopcontrol module 210 may suspend operations and/or provide an error valuewhere system conditions indicate that observed NO_(x) downstream 230 ofthe second SCR catalyst is actually NH₃—for example where increasing thereductant doser command provides corresponding increased observed NO_(x)rather than decreased observed NO_(x). Additionally or alternatively,the outer loop control module 210 suspends operations where systemconditions otherwise indicate that significant NH₃ slip is expecteddownstream of the second SCR catalyst. The outer loop control module 210may perform all operations from a NOx sensor reading downstream of athird or subsequent SCR catalyst where present, from downstream of allof the present SCR catalysts, but in any case downstream of at least twoSCR catalysts, where at least one of the two SCR catalysts is downstreamfrom the NH₃ sensor.

In certain embodiments, the time response adjustment module 214determines a first rate of change that is a time derivative of thepresent amount of NH₃ 232 in the exhaust conduit between the first SCRcatalyst and the second SCR catalyst. The dosing control module 208provides the reductant doser command 226 further in response to thefirst rate of change 232. The utilization of the first rate of change232 in determining the reductant doser command 226 improves theresponsiveness of the controller 124 to transient events. In a furtherembodiment, the time response adjustment module 214 further determines asecond rate of change 234 that is a time derivative of an amount ofNO_(x) downstream of the second SCR catalyst, and the dosing controlmodule 208 further provides the reductant doser command 226 in responseto the second rate of change 234.

The time response adjustment module 214 may determine a product 236 ofthe first rate of change 232 and the second rate of change 234, and thedosing control module 208 provides the reductant doser command 226further in response to the product 236. In a still further embodiment,the time response adjustment module 214 determines a rate-basedadjustment amount 238 as a function of the product 236, and the dosingcontrol module 208 further provides the reductant doser command 226 inresponse to the rate-based adjustment amount 238. The function of theproduct may be a nonlinear response. In certain embodiments, thefunction of the product provides a shallow (or low gain) response to anegative product 236, and a steep (or high gain) response to a positiveproduct 236. A negative product 234 is indicative of a falling NH₃amount or a falling NO_(x) amount. A positive product is indicative ofboth the NH₃ amount and the NO_(x) amount rising or falling together.Any selected response may be captured within the function of theproduct, and may be stored as a polynomial or other mathematicalfunction, as a lookup table, or by other function storing methodunderstood in the art. The time response adjustment module 214 mayfurther limit the rate-based adjustment amount 238 to a proportion of anamount of NO_(x) upstream of the first SCR catalyst.

In certain embodiments, a NO_(x) trimming module 212 further determinesan adjusted downstream NO_(x) amount 222 in response to an amount of NH₃downstream 228 of the second SCR catalyst and an amount of NO_(x)downstream 230 of the second SCR catalyst. The adjusted downstreamNO_(x) amount 222 may be limited to a proportion of an amount of NO_(x)upstream 244 of the first SCR catalyst, and/or an adjustment of thereductant doser command 226 may be limited to a proportion of the amountof NO_(x) upstream 244 of the first SCR catalyst. For example, the NOxtrimming module 212 may cap the adjusted downstream NO_(x) adjustmentamount 222 to the amount of NO_(x) upstream 244 of the first SCRcatalyst (i.e. the proportion=100%), to half the the amount of NO_(x)upstream 244 of the first SCR catalyst, one-quarter, one-tenth, or less.

In certain embodiments, if the amount of NO_(x) downstream 230 of thesecond SCR catalyst exceeds a threshold with the full availabledownstream NO_(x) adjustment amount applied, a fault or other failureindicator may be enabled. In certain embodiments, a fault or failure maybe indicated if the adjusted downstream NO_(x) amount 222 exceeds athreshold value, even if a required downstream NO_(x) can be acceptablyachieved or the adjustment amount from the adjusted downstream NO_(x)amount 222 is not saturated. The rate-based adjustment amount 238 may becapped in a similar manner to the adjusted downstream NO_(x) amount 222.The adjustments may likewise be capped together in a single operation,and/or a final output of all of the adjustments combined may be capped.

In certain embodiments, the dosing control module 208 further providesthe reductant doser command 226 in response to the first rate of change232, the second rate of change 234, the adjusted downstream NO_(x)amount 222, and/or the rate-based adjustment amount 238 by adjusting oneof the reductant doser command 226 and the ammonia reference target 216.For example, the dosing control module 208 may adjust the reductantdoser command 226 to provide an adjusted amount of reductant accordingto the adjustments from the rate-based adjustment amount 238 and theadjusted downstream NO_(x) amount 222. In another example, the dosingcontrol module 208 may modify the ammonia reference target 216, andprovide the reductant doser command 226 consistent with the adjustedammonia reference target 216. Any other adjustment mechanism understoodin the art that results in an adjusted reductant doser command iscontemplated herein.

Referencing FIG. 3, a schematic control diagram 300 is shownillustrating certain operations of an exemplary time response adjustmentmodule. The time response adjustment module receives a mid-catalyst NH₃amount 242, and determines a time derivative 302 of the mid-catalyst NH₃amount 242. The time derivative 302 may be continuous, discrete, or anyother type of mathematical description of a rate of change with time.The time derivative 302 may further be filtered, averaged, or receiveother processing to reduce noise spikes. The time response adjustmentmodule similarly determines a time derivative 304 of a NO_(x) amountdownstream 230 of the second SCR catalyst.

The time response adjustment module determines a product 236 of the timederivatives (the first rate of change and the second rate of change) atoperation 306, and determines a rate-based adjustment amount 238 as afunction of the product. In the example, a nonlinear lookup table 308provides an adjustment value as a function of the product. The nonlinearlookup table 308 provides a low-gain output on the negative product 236side, and a high-gain output on the positive product 236 side, althoughany relationship of a rate-based adjustment amount 238 as a function ofthe product 236 is contemplated herein.

The time response adjustment module further determines an NH₃ amountdownstream 230 of the second SCR catalyst, and determines an adjusteddownstream NO_(x) amount 222 by subtracting the amount of NH₃ downstream228 from the observed NO_(x) amount. It is noted in FIG. 3 that theobserved NO_(x) amount 230, rather than the adjusted downstream NO_(x)amount 222, is used for the second rate of change 304, but either valuemay be utilized. The target amount of NO_(x) downstream of the secondSCR catalyst (NO_(x) reference target 220 or tailpipe NO_(x) target 220)is subtracted from the adjusted downstream NO_(x) amount 222, providinga value that is consistent with an excess downstream NO_(x) amount 224.

The tailpipe NO_(x) target 220 may be determined according to therequired emissions for the application, from current operatingconditions, and/or according to a predetermined value selected by thecontrol designer. The excess downstream NO_(x) amount 224 is filteredand a gain applied in operation 310. The gain may be unity, astoichiometric fraction of reductant to NO_(x), or a selected percentageof the stoichiometric fraction such that the system is responsive butdoes not try to compensate for the entire error through the downstreamNO_(x) feedback mechanism. The filtering of the NO_(x) error downstreammay have a long time constant where the downstream NO_(x) feedbackmechanism is correcting for long-term catalyst degradation. The longtime constant may be an hour, a day, a week, or longer. However, a timeconstant of around a minute or even faster will generally not introducenoise into the operations of the time response adjustment module.Further, the downstream NO_(x) feedback mechanism may be utilized aspart of the general NO_(x) control, not just for degradation control(e.g. where a feedforward ammonia reference target based on models ofthe engine out NO_(x) is not sufficient in a sensitive application, orwhere an upstream NO_(x) sensor is in a fault condition or presentlyunavailable), and therefore an appropriate time constant will beunderstood to one of skill in the art based on the responsibility of thedownstream NO_(x) feedback mechanism.

In the example, the rate-based adjustment amount 238, the downstreamNO_(x) adjustment amount from the operation 310, and the feedforward NH₃reference target 216 are combined to generate an mid-catalyst NH₃reference target 314 for the amount of NH₃ between the first SCRcatalyst and the second SCR catalyst. The total adjustment, or theindividual adjustments, may be capped and the cap may be a proportion ofthe inlet NO_(x) amount 244 to the first SCR catalyst. The adjustmentsmay be added together, averaged, or a greater of the two adjustments maybe applied.

The descriptions which follow provide illustrative embodiments ofperforming procedures for controlling an SCR aftertreatment system.Operations illustrated are understood to be exemplary only, andoperations may be combined or divided, and added or removed, as well asre-ordered in whole or part, unless stated explicitly to the contraryherein. Certain operations illustrated may be implemented by a computerexecuting a computer program product on a computer readable medium,where the computer program product comprises instructions causing thecomputer to execute one or more of the operations, or to issue commandsto other devices to execute one or more of the operations.

An exemplary procedure includes an operation to determine an ammonia(NH₃) reference target that is a target amount of NH₃ in an exhaustconduit between a first selective catalytic reduction (SCR) catalyst anda second SCR catalyst. The procedure further includes an operation todetermine a present amount of NH₃ in the exhaust conduit between thefirst SCR catalyst and the second SCR catalyst, and an operation todetermine an NH₃ error term in response to the NH₃ reference target andthe present amount of NH₃. The procedure further includes an operationto determine an amount of NO_(x) downstream of the second SCR catalyst,and an operation to adjust one of the NH₃ reference target and areductant doser command in response to the amount of NO_(x) downstreamof the second SCR catalyst. The method further includes an operation toprovide providing the reductant doser command in response to the NH₃error term.

In certain embodiments, the procedure further includes an operation todetermine a rate of change of the present amount of NH₃, and further toadjust the one of the NH₃ reference target and the reductant dosercommand in response to the rate of change. In a further embodiment, theprocedure includes an operation to determine a rate of change of theamount of NO_(x) downstream of the second SCR catalyst, and further toadjust the one of the NH₃ reference target and the reductant dosercommand in response to the rate of change of the amount of NO_(x)downstream of the second SCR catalyst.

An exemplary procedure further includes an operation to determine a rateof change of the amount of NO_(x) downstream of the second SCR catalyst,and an operation to adjust the one of the NH₃ reference target and thereductant doser command in response to the rate of change of the amountof NO_(x) downstream of the second SCR catalyst. A further exemplaryprocedure includes an operation to determine a product of the rate ofchange of the present amount of NH₃ and the rate of change of the amountof NO_(x) downstream of the second SCR catalyst, and to further adjustthe one of the NH₃ reference target and the reductant doser command inresponse to the product.

A still further exemplary procedure includes an operation to determine arate-based adjustment amount as a function of the product, and furtherto adjust the one of the NH₃ reference target and the reductant dosercommand in response to the rate-based adjustment amount. A still furtherembodiment of the procedure includes an operation to limit therate-based adjustment amount to a proportion of an amount of NO_(x)upstream of the first SCR catalyst. An exemplary procedure furtherincludes an operation to limit the proportion to an amount less thanone-half of the amount of NO_(x) upstream of the first SCR catalyst.

Another exemplary procedure further includes an operation to determinean amount of NH₃ downstream of the second SCR catalyst, and an operationto adjust the one of the NH₃ reference target and the reductant dosercommand in response to the amount of NH₃ downstream of the second SCRcatalyst. A further exemplary procedure includes an operation todetermine an adjusted downstream NO_(x) amount by subtracting the amountof NH₃ downstream of the second SCR catalyst from the amount of NO_(x)downstream of the second SCR catalyst, and further to adjust the one ofthe NH₃ reference target and the reductant doser command in response tothe adjusted downstream NO_(x) amount. A still further exemplaryprocedure includes an operation to determine an excess downstream NO_(x)amount in response to the adjusted downstream NO_(x) amount and a NO_(x)reference target, and further to adjust the one of the NH₃ referencetarget and the reductant doser command in response to the excessdownstream NO_(x) amount.

Another exemplary procedure is described following. The exemplaryprocedure includes an operation to interpret an NH₃ reference targetthat is a target amount of NH₃ present at a mid-bed position between twoselective catalytic reduction (SCR) catalysts, an operation to interpretan amount of NO_(x) downstream of the SCR catalysts, and an operation toadjust the NH₃ reference target in response to the amount of NO_(x)downstream. The procedure further includes an operation to inject anamount of urea upstream of the SCR catalysts in response to the adjustedNH₃ reference target. In certain further embodiments, the procedureincludes an operation to interpret a rate of change of the amount ofNO_(x) downstream of the SCR catalysts, and/or to interpret a rate ofchange of an amount of NH₃ between the SCR catalyst. The procedurefurther includes an operation to adjust the NH₃ reference target inresponse to one or both of the rates of change. An exemplary procedurefurther includes an operation to determine a product of the rate ofchange of the amount of NO_(x) downstream of the SCR catalysts and therate of change of the amount of NH₃ between the SCR catalysts, and tofurther adjust the NH₃ reference target in response to the product ofthe rates of change.

In certain embodiments, the procedure further includes an operation toperform the adjusting by reducing the NH₃ reference target in responseto the product being negative, and an operation to increase the NH₃reference target in response to the product being positive. An exemplaryprocedure further includes the operation to increase having a highergain than the operation to reduce. In certain embodiments, the procedureincludes an operation to limit the adjusting to a proportion of anamount of NO_(x) upstream of the SCR catalysts.

An exemplary procedure further includes an operation to interpret anamount of NH₃ downstream of the SCR catalysts, an operation to determinean adjusted amount of NO_(x) downstream of the SCR catalysts in responseto the amount of NH₃ downstream of the SCR catalysts, and the operationto adjust the NH₃ reference target is further in response to theadjusted amount of NO_(x) downstream of the SCR catalysts. A furtherexemplary procedure includes an operation to interpret a NO_(x)reference target including a target amount of NOx present at a positiondownstream of the SCR catalysts, an operation to determine an excessdownstream NO_(x) amount in response to the adjusted amount of NO_(x)downstream of the SCR catalysts and the target amount of NO_(x), andoperation to adjust the NH₃ reference target is further in response tothe excess downstream NO_(x) amount. A still further exemplary procedureincludes an operation to filter the excess downstream NO_(x) amount witha filter having a time constant of at least 10 seconds. In certainembodiments, the filter includes a time constant of at least 100 secondsand/or at least 1000 seconds.

As is evident from the figures and text presented above, a variety ofembodiments according to the present invention are contemplated.

An exemplary set of embodiments is a method including determining anammonia (NH₃) reference target comprising a target amount of NH₃ in anexhaust conduit between a first selective catalytic reduction (SCR)catalyst and a second SCR catalyst, determining a present amount of NH₃in the exhaust conduit between the first SCR catalyst and the second SCRcatalyst, and determining an NH₃ error term in response to the NH₃reference target and the present amount of NH₃. The method furtherincludes determining an amount of NO_(x) downstream of the second SCRcatalyst, and adjusting one of the NH₃ reference target and a reductantdoser command in response to the amount of NO_(x) downstream of thesecond SCR catalyst. The method further includes providing the reductantdoser command in response to the NH₃ error term.

In certain embodiments, the method further includes determining a rateof change of the present amount of NH₃, and further adjusting the one ofthe NH₃ reference target and the reductant doser command in response tothe rate of change. In a further embodiment, the method includesdetermining a rate of change of the amount of NO_(x) downstream of thesecond SCR catalyst, and further adjusting the one of the NH₃ referencetarget and the reductant doser command in response to the rate of changeof the amount of NO_(x) downstream of the second SCR catalyst.

An exemplary method includes determining a rate of change of the amountof NO_(x) downstream of the second SCR catalyst, and further adjustingthe one of the NH₃ reference target and the reductant doser command inresponse to the rate of change of the amount of NO_(x) downstream of thesecond SCR catalyst. A further exemplary method includes determining aproduct of the rate of change of the present amount of NH₃ and the rateof change of the amount of NO_(x) downstream of the second SCR catalyst,and further adjusting the one of the NH₃ reference target and thereductant doser command in response to the product. A still furtherexemplary method includes determining a rate-based adjustment amount asa function of the product, and further adjusting the one of the NH₃reference target and the reductant doser command in response to therate-based adjustment amount. A still further embodiment of the methodincludes limiting the rate-based adjustment amount to a proportion of anamount of NO_(x) upstream of the first SCR catalyst. An exemplary methodincludes limiting the proportion to an amount less than one-half of theamount of NO_(x) upstream of the first SCR catalyst.

Another exemplary method includes determining an amount of NH₃downstream of the second SCR catalyst, and further adjusting the one ofthe NH₃ reference target and the reductant doser command in response tothe amount of NH₃ downstream of the second SCR catalyst. A furtherexemplary method includes determining an adjusted downstream NO_(x)amount by subtracting the amount of NH₃ downstream of the second SCRcatalyst from the amount of NO_(x) downstream of the second SCRcatalyst, and further adjusting the one of the NH₃ reference target andthe reductant doser command in response to the adjusted downstreamNO_(x) amount. A still further exemplary method includes determining anexcess downstream NO_(x) amount in response to the adjusted downstreamNO_(x) amount and a NO_(x) reference target, and further adjusting theone of the NH₃ reference target and the reductant doser command inresponse to the excess downstream NO_(x) amount.

Another exemplary set of embodiments is an apparatus including anammonia (NH₃) target module that determines an NH₃ reference target,where the NH₃ reference target is a target amount of NH₃ in an exhaustconduit between a first selective catalytic reduction (SCR) catalyst anda second SCR catalyst. The apparatus includes an NH₃ determinationmodule that determines a present amount of NH₃ in the exhaust conduitbetween the first SCR catalyst and the second SCR catalyst, and an NH₃error module that determines an NH₃ error term in response to the NH₃reference target and the present amount of NH₃. The apparatus furtherincludes a dosing control module that provides a reductant doser commandin response to the NH3 error term, and an outer loop control module thatdetermines an amount of NO_(x) downstream of the second SCR catalyst,and that adjusts one of the NH₃ reference target and the reductant dosercommand in response to the amount of NO_(x) downstream of the second SCRcatalyst.

An exemplary apparatus further includes an NH₃ time response adjustmentmodule that determines at least one of a rate of change of the presentamount of NH₃ and a rate of change of the amount of NO_(x) downstream ofthe second SCR catalyst, and that further adjusts the one of the NH₃reference target and the reductant doser command in response to the atleast one rate of change.

Another exemplary apparatus includes an NH₃ time response adjustmentmodule that determines a product of a rate of change of the presentamount of NH₃ and a rate of change of the amount of NO_(x) downstream ofthe second SCR catalyst, and to further adjust the one of the NH₃reference target and the reductant doser command in response to theproduct. A further exemplary apparatus includes the NH₃ time responseadjustment module further determining a rate-based adjustment amount asa function of the product, and further adjusting the one of the NH₃reference target and the reductant doser command in response to therate-based adjustment amount. In a further embodiment, the NH₃ timeresponse adjustment module further limits the rate-based adjustmentamount to a proportion of an amount of NO_(x) upstream of the first SCRcatalyst. Additionally or alternatively, the function of the product isa non-linear function having a low gain in a negative region of theproduct and a high gain in a positive region of the product.

In certain embodiments, the apparatus includes a NO_(x) trimming modulethat determines an adjusted downstream NO_(x) amount in response to theamount of NO_(x) downstream of the second SCR catalyst and an amount ofNH₃ downstream of the second SCR catalyst, and further adjusts the oneof the NH₃ reference target and the reductant doser command in responseto the adjusted downstream NO_(x) amount. In further embodiments, theNO_(x) trimming module further determines an excess downstream NO_(x)amount in response to the adjusted downstream NO_(x) amount and a NO_(x)reference target, and to further adjust the one of the NH₃ referencetarget and the reductant doser command in response to the excessdownstream NO_(x) amount.

Another exemplary set of embodiments is a system including an internalcombustion engine, an exhaust conduit fluidly coupled to the internalcombustion engine, a first selective catalytic reduction (SCR) catalystfluidly coupled to the exhaust conduit, a second SCR catalyst fluidlycoupled to the exhaust conduit at a position downstream of the first SCRcatalyst, an ammonia sensor operationally coupled to the exhaust conduitat a position between the first SCR catalyst and the second SCRcatalyst, and a reductant doser operationally coupled to the exhaustconduit at a position upstream of the first SCR catalyst.

The system further includes a controller having an ammonia (NH₃) targetmodule that determines an NH₃ reference target. The NH₃ reference targetis a target amount of NH₃ in an exhaust conduit between the first SCRcatalyst and the second SCR catalyst. The controller further includes anNH₃ determination module that determines a present amount of NH₃ in theexhaust conduit between the first SCR catalyst and the second SCRcatalyst, an NH₃ error module that determines an NH₃ error term inresponse to the NH₃ reference target and the present amount of NH₃, anda dosing control module that provides a reductant doser command inresponse to the NH₃ error term. The controller further includes an outerloop control module that determines an amount of NO_(x) downstream ofthe second SCR catalyst, and adjusts one of the NH₃ reference target andthe reductant doser command in response to the amount of NO_(x)downstream of the second SCR catalyst. In certain embodiments, the outerloop control module further adjusts the one of the ammonia referencetarget and the reductant doser command only when the amount of NO_(x)downstream of the second SCR catalyst exceeds a threshold.

Another exemplary set of embodiments is a method including interpretingan NH₃ reference target comprising a target amount of NH₃ present at amid-bed position between two selective catalytic reduction (SCR)catalysts, interpreting an amount of NO_(x) downstream of the SCRcatalysts, adjusting the NH₃ reference target in response to the amountof NO_(x) downstream, and injecting an amount of urea upstream of theSCR catalysts in response to the adjusted NH₃ reference target. Incertain further embodiments, the method includes interpreting a rate ofchange of the amount of NO_(x) downstream of the SCR catalysts, and/orinterpreting a rate of change of an amount of NH₃ between the SCRcatalyst, and where the adjusting inlcudes adjusting the NH₃ referencetarget in response to one or both of the rates of change. An exemplarymethod further includes determining a product of the rate of change ofthe amount of NO_(x) downstream of the SCR catalysts and the rate ofchange of the amount of NH₃ between the SCR catalysts, and where theadjusting includes adjusting the NH₃ reference target in response to theproduct of the rates of change.

In certain embodiments, the method further includes performing theadjusting by reducing the NH₃ reference target in response to theproduct being negative, and increasing the NH₃ reference target inresponse to the product being positive. An exemplary method includes theincreasing having a higher gain than the reducing. In certainembodiments, the method includes limiting the adjusting to a proportionof an amount of NO_(x) upstream of the SCR catalysts.

An exemplary method further includes interpreting an amount of NH₃downstream of the SCR catalysts, determining an adjusted amount ofNO_(x) downstream of the SCR catalysts in response to the amount of NH₃downstream of the SCR catalysts, and where the adjusting is further inresponse to the adjusted amount of NO_(x) downstream of the SCRcatalysts. A further exemplary method includes interpreting a NO_(x)reference target including a target amount of NO_(x) present at aposition downstream of the SCR catalysts, determining an excessdownstream NO_(x) amount in response to the adjusted amount of NO_(x)downstream of the SCR catalysts and the target amount of NO_(x), andwhere the adjusting is further in response to the excess downstreamNO_(x) amount. A still further exemplary method includes filtering theexcess downstream NO_(x) amount with a filter having a time constant ofat least 10 seconds. In certain embodiments, the filter includes a timeconstant of at least 100 seconds and/or at least 1000 seconds.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly certain exemplary embodiments have been shown and described andthat all changes and modifications that come within the spirit of theinventions are desired to be protected. In reading the claims, it isintended that when words such as “a,” “an,” “at least one,” or “at leastone portion” are used there is no intention to limit the claim to onlyone item unless specifically stated to the contrary in the claim. Whenthe language “at least a portion” and/or “a portion” is used the itemcan include a portion and/or the entire item unless specifically statedto the contrary.

1. A method, comprising: determining an ammonia (NH₃) reference targetcomprising a target amount of NH₃ in an exhaust conduit between a firstselective catalytic reduction (SCR) catalyst and a second SCR catalystlocated in the exhaust conduit; determining a present amount of NH₃ inthe exhaust conduit between the first SCR catalyst and the second SCRcatalyst with an ammonia sensor; determining an NH₃ error term inresponse to the NH₃ reference target and the present amount of NH₃;determining a reductant closer command in response to the NH₃ errorterm; determining an amount of NO_(x) downstream of the second SCRcatalyst, and adjusting one of the NH₃ reference target and a reductantdoser command in response to the amount of NO_(x) downstream of thesecond SCR catalyst; and injecting, with a reductant injector, an amountof reductant in response to the adjusted one of the NH₃ reference tametand the reductant doser command.
 2. (canceled)
 3. The method of claim 2,further comprising determining a rate of change of the amount of NO_(x)downstream of the second SCR catalyst, and further adjusting the one ofthe NH₃ reference target and the reductant doer conunand in response tothe rate of change of the amount of NO_(x) downstream of the second SCRcatalyst. 4.-8 (canceled)
 9. The method of claim 1, further determiningan amount of NH₃ downstream of the second SCR catalyst, and furtheradjusting the one of the NH₃ reference target and the reductant closercommand in response to the amount of NH₃ downstream of the second SCRcatalyst.
 10. The method of claim 9, further comprising determining anadjusted downstream NO_(x) amount by subtracting the amount of NH₃downstream of the second SCR catalyst from the amount of NO_(x)downstream of the second SCR catalyst, and further adjusting the one ofthe NH₃ reference target and the reductant doser command in response tothe adjusted downstream NO_(x) amount.
 11. The method of claim 10,further comprising determining an excess downstream NO_(x) amount inresponse to the adjusted downstream NO_(x) amount and a NO_(x) referencetarget, and further adjusting the one of the NH₃ reference target andthe reductant closer command in response to the excess downstream NO_(x)amount.
 12. An apparatus, comprising: an electronic controller operableconnected to a reductant closer, the electronic controller including aplurality of modules that are implemented as at least one of hardwareand instructions on a computer readable medium, the plurality of modulesincluding: an ammonia (NH₃) target module structured to determine an NH₃reference target comprising a target amount of NH₃ in an exhaust conduitbetween a first selective catalytic reduction (SCR) catalyst and asecond SCR catalyst wherein the NH₃ reference target is the targetamount of NH₃ that occurs at a mid-bed position between the first SCRcatalyst and the second SCR catalyst in response to injection of atargeted amount of reductant upstream of the first SCR catalyst; an NH₃determination module structured to determine a present amount of NH₃ inthe exhaust conduit between the first SCR catalyst and the second SCRcatalyst in response to an output from an ammonia sensor that defines aseparation point between the first SCR catalyst and the second SCRcatalyst at the mid-bed position; NH₃an error module structured todetermine an NH₃ error term in response to the NH₃ reference target andthe present amount of NH₃; a dosing control module structured to providea reductant closer command in response to the NH₃ error term; and anouter loop control module structured to determine an amount of NO_(x)downstream of the second SCR catalyst, and to adjust one of the NH₃reference target and the reductant closer command in response to theamount of NO_(x) downstream of the second SCR catalyst and control thereductant (loser to inject reductant in response to the adjusted one ofthe NH₃ reference target and the reductant closer command.
 13. Theapparatus of claim 12, further comprising an NH₃ time responseadjustment module structured to determine at least one of a rate ofchange of the present amount of NH₃ and a rate of change of the amountof NO_(x) downstream of the second SCR catalyst, and to further adjustthe one of the NH₃ reference target and the reductant (loser command inresponse to the at least one rate of change.
 14. The apparatus of claim12, further comprising an NH₃ time response adjustment module structuredto determine a product of a rate of change of the present amount of NH₃and a rate of change of the amount of NO_(x) downstream of the secondSCR catalyst, and to further adjust the one of the NH₃ reference targetand the reductant Closer command in response to the product.
 15. theapparatus of claim 14, wherein the NH₃ time response adjustment moduleis further structured to determine a rate-based adjustment amount as afunction of the product, and to further adjust the one of the NH₃reference target and the reductant doser command in response to therate-based adjustment amount.
 16. The apparatus of claim 15, wherein theNH₃ time response adjustment module is further structured to limit therate-based adjustment amount to a proportion of an amount of NO_(x)upstream of the first SCR catalyst.
 17. The apparatus of claim 15,wherein the function of the product comprises a non-linear functionhaving a low gain in a negative region of the product and a high gain ina positive region of the product.
 18. The apparatus of claim 12, furthercomprising a NO_(x) trimming module structured to determine an adjusteddownstream NO_(x) amount in response to the amount of NO_(x) downstreamof the second SCR catalyst and an amount of NH₃ downstream of the secondSCR catalyst, and to further adjust the one of the NH₃ reference targetand the reductant closer command in response to the adjusted downstreamNO_(x) amount.
 19. The apparatus of claim 18, wherein the NO_(x)trimming module is further structured to determine an excess downstreamNO_(x) amount in response to the adjusted downstream NO_(x) amount and aNO_(x) reference target, and to further adjust the one of the NH₃reference target and the reductant doser command in response to theexcess downstream NO_(x) amount. 20-21. (canceled)
 22. A method,comprising: interpreting an NH₃ reference target comprising a targetamount of NH₃ present at a mid-bed position between two selectivecatalytic reduction (SCR) catalysts, wherein the NH3 reference target isthe target amount of NH₃ that occurs at a mid-bed position between theSCR catalysts in response to injection of a targeted amount of reductantupstream of the SCR catalysts; interpreting an amount of NO_(x)downstream of the SCR catalysts and a present amount of NH₃ in theexhaust conduit between the SCR catalysts in response to an output fromammonia sensor that defines a separation point between the SCR catalystsat the mid-bed position; adjusting the NH₃ reference target in responseto the amount of NO_(x) downstream of the SCR catalysts, the presentamount of NH₃, and the NH₃ reference target; and injecting, with areductant injector, an amount of urea upstream of the SCR catalysts inresponse to the adjusted NH₃ reference target. 23-27. (canceled)
 28. Themethod of claim 22, further comprising interpreting an amount of NH₃downstream of the SCR catalysts, determining an adjusted amount ofNO_(x) downstream of the SCR catalysts in response to the amount of NH₃downstream of the SCR catalysts, and wherein the adjusting is further inresponse to the adjusted amount of NO_(x) downstream of the SCRcatalysts.
 29. The method of claim 28, further comprising interpreting aNO_(x) reference target comprising a target amount of NO_(x) present ata position downstream of the SCR catalysts, determining an excessdownstream NO_(x) amount in response to the adjusted amount of NO_(x)downstream of the SCR catalysts and the target amount of NO_(x) andwherein the adjusting is further in response to the excess downstreamNO_(x) amount.
 30. The method of claim 29, further comprising filteringthe excess downstream NO_(x) amount with a filter having a time constantof at least 10 seconds.
 31. The method of claim 29, further comprisingfiltering the excess downstream NO_(x) amount with a filter having atime constant of at least 100 seconds,
 32. The method of claim 29,further comprising filtering the excess downstream NO_(x) amount with afilter having a time constant of at least 1000 seconds.